Wikipedia says Riddhima will cheat Siddhant and will go around with Armaan. Wikipedia article states the following-
Siddhant finds out that Armaan is back. But before he can tell Riddhima, Riddhima takes Sid to a romantic date. They dance and kiss. Riddhima and Siddhant consumate their relationship and Riddhima promises to be with Sid forever. But when she knows of Armaan's return through Dr. Keerti, she goes to meet him. They hug each other and kiss passionately. Siddhant is busy in Sanjeevani. So Riddhima takes advantage of the situation and takes Armaan to her nearby house. There they make love for the first time. Riddhima later feels sad because she cheated Sid. She decides to be a good wife to rectify her one night stand with Armaan. But soon Riddhima finds out that she is pregnant. She doesn't know who is the father because she slept with both Armaan and Siddhant simultaneously. Siddhant is really happy to know about her pregnancy. But Riddhima wants to reveal that she cheated him. But Armaan stops her from doing so and asks to go for a DNA test to find the real father. Armaan promises that if he is the real father then, Riddhima should divorce Sid and marry Armaan. But Riddhima is confused whom to choose.
http://en.wikipedia.org/wiki/Dill_Mill_Gayye_(season_2)#Storyline
Web Series
Kabhi Neem Neem Kabhi Shahad Shahad
